Daniele B. Malesani (DTU Space), Antonio de Ugarte Postigo (HETH,
IAA-CSIC; DARK/NBI), Luca Izzo (DARK/NBI), Jens Hjorth (DARK/NBI), D.
Alexander Kann (HETH, IAA-CSIC), Christina C. Thoene (HETH, IAA-CSIC),
Martin A. Guerrero (IAA-CSIC), report on behalf of a larger collaboration:
We observed the field of GRB 200729A (Evans et al., GCN 28165) with the
Nordic Optical Telescope equipped with the ALFOSC imager. Observations
were carried out in the r and z filters, and started on 2020 Jul 29.911
UT (2.2 hr after the trigger), as soon as visibility allowed.
Within the XRT position of the candidate afterglow (Osborne et al., GCN
28168), we detect no new sources compared to archival Pan-STARRS images.
The 3-sigma limiting magnitudes are:
r > 22.5 AB
z > 22.0 AB
As noted by Evans et al. (GCN 28165), the location of the X-ray source
lies on top of the nearby galaxy NGC 4242, whose distance is ~5.5 Mpc
(from Simbad, which however presents values up to 10 Mpc). We note that,
if the GRB is associated with NGC 4242, our limit would correspond to an
absolute magnitude M_r > -6.2, which, in lack of significant extinction,
provides a deep limit to the GRB optical luminosity. For example, the
earliest detection of SN 2008D (3.5 hr after the outburst; Soderberg et
al. 2008, Nat, 453, 469) had an absolute U-band magnitude M(U) = -12.3
(even before correcting for the significant extinction).
At 5.5 Mpc, the gamma-ray fluence of 8*10^-7 erg cm^-2 (taken from
https://gcn.gsfc.nasa.gov/notices_s/984929/BA/) would correspond to
3*10^45 erg - fainter than the low-luminosity events GRB 980425, GRB
170817A, and the X-ray output of SN 2008D.
While the visibility of this target from La Palma is now over, we
encourage further observations from other locations.